German Patent Disclosure DE 199 10 970 A1 discloses a fuel injection system having a pressure boosting unit, which is located between a pressure reservoir and a nozzle chamber and whose pressure chamber communicates with the nozzle chamber via a pressure line. A bypass line connected to the pressure reservoir is also provided. The bypass line communicates directly with the pressure line. The bypass line can be used for a pressurized injection and is located parallel to the pressure chamber, so that the bypass line is passable, regardless of the motion and position of a displaceable pressure fluid in the pressure boosting unit. This makes greater flexibility in terms of the injection possible.
German Patent Disclosure DE 101 23 911.4 relates to a fuel injection system with a pressure boosting device. A fuel injection system for internal combustion engines includes a fuel injector, which can be supplied from a high-pressure fuel source and has a pressure boosting device. The pressure boosting device includes a movable piston, which divides a chamber connected to the high-pressure fuel source from a high-pressure chamber communicating with the injector. The high-pressure chamber communicates with a differential pressure chamber via a fuel line, so that the high-pressure chamber can be filled with fuel via the differential pressure chamber of the pressure boosting device. The triggering of the fuel injection system with the pressure boosting device known from DE 101 23 911.4 is effected via a pressure relief of the differential pressure chamber of the pressure boosting device. The systems known from DE 199 10 970 A1 and DE 101 23 911.4, include a stroke-controlled fuel injector. Each fuel injector is assigned a pressure booster, for elevating the injection pressure as needed. The triggering of the pressure boosting device is effected via a simple 2/2-way valve and leads to reduced depressurization losses, since the differential pressure chamber of the pressure boosting device is pressure-relieved for its actuation. Moreover, these systems make it possible to perform multiple injections and to shape the injection course flexibly.
The use of a pressure boosting device in a fuel injection system that includes a common rail leads to a greatly increased fuel quantity demand per fuel injector within the injection system. For a high-pressure pumping unit, the result is an increased pumping quantity at a reduced pressure level. For a low-pressure pump, the pumping quantity also increases. The pressure level of the low-pressure pumping unit, however, does not decrease, since good filling of the pump chambers of the high-pressure pumping unit and exact meterability of the pumping quantity by the metering unit in the fuel system must be assured. Designing the prefeed pump for the large-quantity flows required in fuel injectors with a pressure booster is therefore a problem. In a fuel injection system with a common rail with an integrated pressure booster, high return quantities occur because of the pressure boosting, and these quantities amount to multiple times the fuel quantity to be injected via the respective fuel injector. In the systems known from the prior art, this fuel quantity is depressurized completely and is delivered to the fuel tank, which is exposed only to atmospheric pressure. The entire quantity demanded by the fuel injection system must then be compressed by the low-pressure pump to the prefeed pressure, to enable filling of the pump chambers of the high-pressure pumping unit.